For non-divers: the lower you go, the less time you have before you absorb too much nitrogen through your skin, which causes you to enter a drunken and even narcotic state. for reference: if you stay at 18m depth you can stay for at least half an hour, whereas at 40m you can't stay longer than a few minutes before it gets at dangerous levels.
Also: at 90m the oxygen becomes toxic, due to the pressure. You breathe in so many oxygen particles in one breath at that pressure, you actually need to mix in various other gasses to counter it.